Wireless communications applications have increased to crowd the available spectrum and drive the need for high isolation between adjacent bands. Portability requirements of mobile communications additionally require a reduction in the size of communications equipment. Filters used in communications devices have been required to provide improved performance using smaller sized components. Efforts have been made to develop new types of resonators, new coupling structures, and new configurations to address these requirements.
Electrically tunable microwave filters have many applications in microwave systems. These applications include local multipoint distribution service (LMDS), personal communication systems (PCS), frequency hopping radio, satellite communications, and radar systems. There are three main kinds of microwave tunable filters, including mechanically, magnetically, and electrically tunable filters. Mechanically tunable filters suffer from slow tuning speed and large size. Compared to mechanically and magnetically tunable filters, electrically tunable filters have the important advantages of small size and fast tuning capability over relatively wide frequency bands. Electrically tunable filters include voltage-controlled tunable dielectric capacitor based tunable filters, and semiconductor varactor based tunable filters. Compared to semiconductor varactor based tunable filters, tunable dielectric capacitor based tunable filters have the merits of lower loss, higher power-handling, and higher IP3, especially at higher frequencies (>10 GHz).
Tunable filters offer communications service providers flexibility and scalability never before accessible. A single tunable filter can replace several fixed filters covering adjacent frequencies. This versatility provides transceiver front end RF tunability in real time applications and decreases deployment and maintenance costs through software controls and reduced component count. Also, fixed filters need to be wide band so that their count does not exceed reasonable numbers to cover the desired frequency plan. Tunable filters, however, are typically narrow band, but they can cover a larger frequency band than fixed filters by tuning the filters over a wide range. Additionally, narrowband filters at the front end are appreciated from the systems point of view, because they provide better selectivity and help reduce interference from nearby transmitters.
Commonly owned U.S. patent application Ser. No. 09/419,126, filed Oct. 15, 1999, and titled “Voltage Tunable Varactors And Tunable Devices Including Such Varactors”, discloses voltage tunable dielectric varactors that operate at room temperature and various devices that include such varactors, and is hereby incorporated by reference.
Commonly owned U.S. patent application Ser. No. 09/457,943, filed Dec. 9, 1999, and titled “Electronic Tunable Filters With Dielectric Varactors”, discloses microstrip filters including voltage tunable dielectric varactors that operate at room temperature, and is hereby incorporated by reference.
Commonly owned U.S. patent application Ser. No. 10/013,265, filed Dec. 10, 2001, and titled “Electrically Tunable Notch Filters” discloses electronically tunable notch filters, and is hereby incorporated by reference.
Combline filters are attractive for use in electronic communications devices. It is well known that combline filters, in general, have a natural transmission zero above its passband. One of the techniques used to reduce the number of resonators is to add cross couplings between non-adjacent resonators to provide transmission zeros. Examples of this approach are shown in U.S. Pat. No. 4,418,324 and 5,543,764.
In some filter applications, a tunable asymmetric response is desirable. There is a need for tunable filters with an asymmetric response.